Most Westerners do not only need a visa to visit Iran, they first need a "Letter of Invitation" to be able to apply for the visa.
Most people get this through travel or special visa agencies, some of them are online only.

What are any trustworthy agencies that do this? Experiences in quality and duration?

This will surely depend a lot on where the person applying is from so you should include that in your question.
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hippietrailNov 17 '11 at 14:14

Even if it does, it would be a good idea to have an answer for various different nationalities. I have a German passport.
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Peter HahndorfNov 17 '11 at 14:18

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Keep in mind the rules from the Stack Exchange FAQ that the site is for specific answers to specific questions and that lists are discouraged. Maybe there's a single answer that works for everyone but if not we should at least be able to find an answer specific for German citizens.
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hippietrailNov 17 '11 at 14:27

I expect that these online visa agencies will be able to get you a visa regardless of the type of passport you hold. So we don't want any 'community wikis' here? On Stack Overflow they are pretty common.
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Peter HahndorfNov 17 '11 at 14:41

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I am pretty sure its much more difficult to get an Iranian visa if you have American passport.
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GrzenioNov 18 '11 at 22:02

5 Answers
5

In your country's Iranian embassy or consulate

Each embassy should contain info about visa application. Note you will probably need authorisation code from tourist agencies (like Key2Persia or Touran Zamin). It does mean an extra fee (~40 EUR). You can access such code in quick mode (max. 10 working days) or standard mode (above 10 working days). I used Key2Persia and I confirm it is working and safe. Authorisation code is needed for an embassy or consulate for final visa application.

I applied for Visa in Polish embassy in Warsaw and post documents via post-office. I contacted embassy via phone and got visa (I managed delivery company to receive my documents from embassy) in 5 working days.

Delivery company cannot be from U.S. as there is an embargo. You cannot explicitly title your cash transfer and you cannot include word Iran on it. There are special ways of payment.

Visa from embassy can be extended.

In other embassies

These are also embassies that have positive feedback from travel blogs around:

You don't have to be a citizen of these countries. You apply for a visa, pay a cash in bank (~70 EUR) and get a visa the same day evening or the next day.

Tehran airport

Such visa is valid for max. 15 days and sometimes it's required to have a Iranian friend name. It can be from Couchsurfing but we don't say it's from there. It's important to have a phone number in which the person can be reached and can confirm our arrival.

Such visa can be extended (despite many says it cannot be) and it costs ~$10.

You can pickup your visa after 3 days Within 25 days at an Iranian embassy or consulate you can receive your visa.

In this case You can apply for a 30 days Iran visa initially.

Why not to consider Visa upon arrival for Iran?

This visa which is granted in 5 International airports upon arrival may seem like the easiest way to receive Iran visa but for the following reasons our incoming staff with years of experience strongly unqualify this type of visa. Below are the most important reasons for you not to consider visa on entry for Iran:

A 5 % possibility of visa rejection always exists.

Your airline may not let you board the plane without a visa.

The duration is not as long as the tourist visa, up to 14 days.

Most likely you will have to stay long hours in the airport for receiving this Iran visa.

Iran Tourist visa in detail:

Iran tourist visa is issued for almost all nationalities. All who are interested in traveling to Iran as groups or individuals can apply for this visa. This visa is also best for family visitation purposes. The duration of this visa can vary from 14 to 30 days based on application. You can apply for max 90 days extension easily in major cities once in Iran.

How you can Picking Up Your Iran Visa?

Travel Agencies will send your Visa authorization number after it is issued in Iran foreign ministry. Your visa authorization number is a 6 or 7 digit number that refers to your specific visa application case. This code is valid for 25 days in the Iran embassy or consulate that you have chosen to pick up your visa at. You can also post your passport. Please choose your pick up location accurately since change in location in usually not possible. Pick up policy, waiting time, and many other factors vary in different embassies and consulate. Some embassies do not oblige applicant to pick up in person other do. Be sure to contact the embassies and consulate beforehand. The visa fee which is apart from our service fee varies based on nationality. Some embassies may issue the visa within 1-2 hours but others may keep your passport for up to a week. Depending on your nationality and pick up choices the regulations are different so it is very important for you to be correctly informed. The procedures can be handled hassle free if you are provided with the correct information and have scheduled for the steps.

If you have trouble arranging your pick up and no time for postal arrangements we arrange for you to pick it up in your arrival port in Iran. Be sure to contact us before planning based on this fact.

Iran tourist visa for U.S. Canadian and British Citizens

The clearance process for these nationalities may take longer than 14 days. I would advise you to consider receiving your Authorization number between 30-45 days after application when planning for your trip to Iran.

U.S. , Canadian, and British Citizens are obliged to have Iranian authorized tour guides accompanying them all throughout their trip.

As I found out, there's an option for citizens of the following countries to get an airport transit visa (for tourists) at IKA airport (Tehran) or some other famous cities for one week which can't be extended:

In general that is a good suggestion but not always possible especially not if you are already traveling and need to get the visa while on the road. One still doesn't meet too many Iranians in hostels.
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Peter HahndorfNov 18 '11 at 9:23

You are right. This is not an advice that can be applied at last minute. Anyways, the goal to making friends should not be to get a visa but to actually get new friends.
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mouvicielNov 18 '11 at 9:38

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So rather than an letter of invitation you need an authorization code from the government which may be difficult to get for a normal Iranian citizen.
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Peter HahndorfNov 19 '11 at 18:32

I just had a chat with an Australian girl who went to Iran last year, the general procedure is like this:

Find an Iranian travel agency to get you an authorization Code from the Iranian Ministry of Foreign Affairs and follow these steps:

Fill out an online form on the web site of the agency

Scan in your passport and email the image to the agency.

Pay them a fee, around 35 Euros. (The Aussie girl had to make a transfer into a German bank account, as you cannot directly transfer money into Iranian accounts)

Tell the agency which consulate you want to use to get the visa.

After having all this the agency will try to get you an authorization code from the Iranian Ministry of Foreign Affairs which it then sends back to you. In our case this only took one day but it may take much longer.

Now you can go to the consulate of your choice or send them the visa application including the authorization code. The processing may take several weeks but our Australian friend had her passport back after five days.

The authorization code is only valid to the chosen consulate and only for 3 months from the date of issue.

This seems to be the procedure for all Nationalities that need a visa for Iran, except for US citizens:

At present the Iranian Foreign Ministry does not allow American
passport holders to travel to Iran independently. Americans are
required to travel on escorted tours; either as part of a tour group,
or a tailor made individual tour. An exact itinerary, to which you
must adhere, is compulsory.

A few years later but I'm finally getting to Iran. I ordered and paid for my authorization code on Feb 24th 2014 and received it today (Apr 8th 2014), this was due to a lengthy holiday period in Iran. So it can take quite a while, try to order it early. Now I'm waiting for the actual visa.
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Peter HahndorfApr 8 '14 at 20:44